Crucible seal



Oct. 18, 1966 N. E. HAMILTON ETAL.

CRUCIBLE SEAL Filed Oct. 26, 1960 FIGJ S R 0 M m m NOBLE E. HAMILTON MARK C.WELCH ATTORNEY environment.

United States Patent 3,279,896 CRUCIELE SEAL Noble E. Hamilton, Belmont, Mass, and Mark C. Welch,

Reeds Ferry, N.H., assigncrs, by mesne assignments, to

International Telephone and Telegraph Corporation,

New York, N.Y., a corporation of Maryland Filed Get. 26, 196%), Sex. Na. @5318 11 Claims. (Cl. 23-601) This invention relates generally to sealing methods and apparatus. More particularly the invention pertains to methods and apparatus for providing a liquid-tight seal around a body passing through an aperture into a vessel containing a liquid.

While the need for such seals arise frequently in connection with a wide variety of chemical and metallurgical processes and apparatus, the invention is deemed at the present time to have particular advantage and utility in its application to the growth of single crystals of semiconductor materials as described in copending application of N. E. Hamilton for United States Letters Patent Serial No. 787,939, filed January 20, 1959 and assigned to the same assignee as the present invention. Accordingly, the present invention is herein described in this particular It will be understood, however, that this is solely by way of example rather than limitation and that many other applications of the inventive principles may manifest themselves to persons skilled in the art.

Briefly stated the crystal growing method and apparatus described in the aforementioned application involves the use of a refractory vessel for containing a melt of the material to be crystallized and for passing a charge bar of such material through an open bore in the bottom of the vessel to feed the melt while a single crystal is pulled from above. This method and apparatus depends on the surface tension of the melt to prevent leakage through the clearance space between the feed aperture and charge bar. Consequently, the maximum permissible clearance is critical, requiring careful preparation of the charge bar to avoid dimensional variations and/or surface irregularities. Reliance solely on surface tension for sealing also places a practical limit on the permissible hydrostatic pressure in the melt which, in turn, requires an appreciable margin of safety to avoid leakage in the event of impulse pressures. 1

The fundamental general object of the present invention is to provide a novel sealing method and apparatus which avoid or mitigate at least one of the problems of the prior art as mentioned above.

A more particular object is the provision of an improved liquid-tight seal for an object passing through an aperture which tolerates appreciable variations in the clearance space to be sealed.

Another object is the provision of an improved liquidtight seal for an object passing through an aperture which is self-forming, self-adjusting and self-renewing.

Still another object is the provision of a novel liquidtight seal for an object passing through an aperture which automatically adjusts to compensate for variations in the clearance space to be sealed and does not impose any undue frictional drag on the relative movement of the parts sealed.

Another object is the provision of novel liquid sealing method-s and apparatus which enable the utilization of surface tension for sealing across greater clearance spaces and/or against higher hydrostatic pressure than could otherwise be sealed by surface tension effects.

A specific object is the provision of improved methods and apparatus for pulling crystals from the melt in which parameters relating to or effected by the cross-sectional dimensions and configuration of the charge material fed Patented Get. 1%, 1966 to the melt and the hydrostatic pressure of the melt are rendered less critical than in comparable prior art methods and apparatus.

These and additional objects of the invention are accomplished by the provision, on the body to be passed throughout the several views and FIGURE 1 is an axial sectional view of apparatus embodying the invention; and

FIGURE 2 is a fragmentary view showing a portion of the structure of FIGURE 1 on an enlarged scale.

Except as modified to incorporate and illustrate the ,present invention the apparatus depicted in FIGURE 1 substantially identical to that shown, described and claimed in the aforementioned application Serial No. 787,939. The apparatus, designated as a whole by reference numeral 10, comprises a crucible member 12 laterally surrounded by a heat-generating element 14 and disposed within an enclosure 16.

The upper end of crucible member 12 is formed with a cup-shaped cavity 18 which serves to contain a melt '20 from which a single crystal 22 is pulled in the manner well-known in the art and referred to as the Czochra'lski method.

The lower end of crucible member 12 contains an openended bore 24 coaxial with and extending into the bottom of cavity 18. A charge" bar 26 of the material to be crystallized is dimensioned to be slidably received in bore 24 so that the bar may be passed vprogressively through the bore and thus replenish :melt 20.

Heat-generating element 14 may be any suitable device for heating the region of cavity 18 which it surrounds. In the illustrated embodiment element 14 is a high frequency electromagnetic induction coil and is connected to a source of high frequency alternating current, not shown. The region of cavity 18 enveloped by coil 14 extends from the bottom of the cavity, i.e., shoulder 28, to a point at a substantial distance below the top of the cavity. Coil 14 may be mounted on crucible 12 or otherwise arranged so that its position relative to the crucible is 'fixed.

The remainder of the structure illustrated in FIGURE 1 is not germane to, nor essential to an understanding of, the present invention and, therefore, need not be described herein. For a complete description of such structure and its functions reference may be had to the aforementioned c-opending application Serial No. 787,939. From the structure thus far described, however, it will be understood that melt 20 in crucible cavity 18 is replenished by means of charge bar 26 which moves progressively into bore 24. Under these circumstances it will be appreciated that it is necessary effectively to seal the clearance space (shown in exaggerated dimensions at 30, FIGURE 2) between bar 26 and the walls of bore 24 to prevent leakage of melt 20' without unduly impeding the movement of the bar through the bore. In the apparatus disclosed in the aforementioned copending application Serial No. 787,939 the required sealing is accomplished by and depends on the characteristic surface tension of melt 20. Inasmuch as the maximum value of surface tension available for scaling is fixed for any particular substance in the crucible, there is a critical relation between the amount of clearance which can be 3 tolerated and the hydrostatic pressure of the melt tending to produce leakage. This in turn necessitates careful control of the configuration of the charge bar to keep surface irregularities to a minimum in both number and degree and to hold variations in size and shape of the bar within close limits.

It will be understood that even a small amount of leakage of melt Ztl into the bore 24 is intolerable inasmuch as the melt solidifies in the cooler regions causing the charge bar to bind in the bore, disrupting entirely the operation of the apparatus.

In accordance with the present invention factors hearing directly on or influencing the efficacy of the seal between the feed bar and the crucible bore are rendered less critical by means of a surface coating 32 of suitable thickness applied to charge 'bar 26 prior to its insertion into bore 24 and adapted to reduce the lateral clearance and participate in the formation of a fluid-tight seal between the bar and the bounding surfaces of the bore. It is to be understood that the thickness of coating 32, like clearance 30, is greatly exaggerated in the drawings for clarity of illustration.

While the chemical identity and physical characteristics of coating 3-2 would vary with the particular apparatus in which the seal is employed, as Well as with the identity of the solid feed material and the liquid into which it is fed, the coatings may be classified according to their mechanism of operation into two general types:

(1) Coatings which entirely supplant, and operate independently of, surface tension of the melt and (2) Coatings which coact with and utilize surface tension effects in the formation of the seal.

In some cases a single coating may be of either type depending upon the particular service conditions.

Both types of coatings have certain characteristics in common: they should be substantially continuous and friable; and they should not melt in service, i.e., they should have a melting point higher than the ambient temperatures to be encountered and where, as in the exemplary embodiment, the liquid sealed against is a molten solid the coating material should have a higher melting point than such solid. In situations Where melting is not involved, the material of both types of coatings should be substantially insoluble in the liquid sealed against. In some cases, it may be required that the coating material be not reactive with and completely insoluble in the liquid so as not to contaminate the liquid.

Turning now to the individual distinguishing characteristics of the respective types of coating materials specified above, in type ll (i.e., where the coating material forms the seal independently of surface tension) it is necessary that the material of the coating be capable of packing to some extent as opposed to being very fluid or plastic in nature. As a specific example of this type of material a coating of finely divided carbon is highly satisfactory on a germanium charge bar fed, in the manner described in conjunction with FIGURE 1, into a melt of germanium. This coating may be applied in any suitable manner but this is conveniently accomplished by exposing the surfaces to a sooty flame.

In operation such a coating is carried on the feed bar until it reaches melt 20, or other liquid to be sealed against. At this point the solid feed bar melts or dissolves in the liquid while the coating, insoluble or infusible at the temperature involved, is disrupted by the liquid pressure :and packed as shown at 34, FIGURE 2, into clearance :space 30 between the bar and the wall of bore 24. This :results in the formation of a continuously self-renewing somewhat undersized (i.e., somewhat smaller than an immediately preceding portion of the bar) the gap which would ordinarily result with a conventional seal does not come into being inasmuch as the coating immediately and continuously compensates for the diminution in the bar dimension. Conversely, the occurrence of an oversized segment in the bar simply packs the accumulated seal material a little tighter or erodes a sufficient quantity of the friable material to achieve precisely the required dimension.

The second type of coating 32 contemplated by the invention is a relatively continuous layer or film on the surface of the bar characterized by appreciable strength as compared to the surface tension of the liquid to be contained but which is weak relative to the desirable forces involved in feeding the coated bar into the aperture to be sealed. The coating is such that upon being disrupted it forms platelets or particles adapted to and capable of lodging in and bridging the clearance space, thus allowing smaller surface radii, i.e., small enough that surface tension is effective to prevent leakage of the liquid.

A particularly successful example of this type of coating is a film of silica applied to germanium feed bars for use in crystallizing apparatus such as described above and illustrated in FIGURE 1. The coating is formed by wetting the germanium bars with ethyl silicate and allowing them to air-dry. Moisture present in the air probably causes some hydrolysis during the formation of the coat ing which probably is hydrated silica.

While there have been described what at present are believed to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed and desired to be secured by United States Letters Patent is:

1. In combination with a method of passing a solid body into a liquid-filled vessel through an aperture located below the liquid surface, the step of applying to said body a friable substantially continuous surface coating adapted to be disrupted in said aperture to form a residue participating in the formation of a seal between said body and vessel, said coating being substantially insoluble and non-reactive in the liquid and having a melting temperature exceeding the temperature of the liquid.

2. A method according to claim 1 wherein said coating is applied by wetting the body with ethyl silicate followed by air-drying.

3. A method of growing single crystals of a fusible solid material from a melt, comprising: preparing a bar of said material; applying to the bar a substantially continuous friable surface coating of a material which is substantially non-sublimable, non-reactive and insoluble in a melt of the material of said bar and has a melting temperature exceeding the temperature of the melt; disposing said bar in a substantially vertical position; inserting the upper end of said bar into an open refractory vessel through an aperture in the bottom wall of said vessel; establishing a molten zone adjacent the upper end of said bar; and removing crystallized material from the molten zone while simultaneously progressively moving said bar relative to said vessel through said aperture and into the molten zone, said coating being continuously disrupted within said aperture during passage of said bar therethrough to form a residue between said bar and said aperture forming a liquid tight seal.

4. A consumable charge bar for insertion through an aperture into a crucible member containing a melt comprising: an elongated bar of charging material having a cross-sectional configuration complemental to that of the aperture to be passed therethrouigh with a predetermined lateral clearance space; and a continuous surface coating on said bar comprising a substance disrupted during passage of said bar through the aperture to form a residue filling said lateral clearnance space to establish a liquid tight seal, said coating having a melting point higher than the temperature of the melt and insoluble therein. 7

5. A charge bar :as claimed in claim 4 wherein said elongated bar is of germanium material for insertion into a germanium melt and said coating comprises finely divided carbon.

6. A charge bar as claimed in claim 4 wherein said elongated bar is of germanium material for insertion into a germanium melt and said coating is formed of silica.

7. The method of passing a solid body into a liquid filled vessel having an aperture in the bottom wall thereof adapted to sli-dably receive the body with a predetermined lateral clearance, which includes the steps of: applying to the body a friable substantially continuous surface coating; and moving the body vertically upward through the aperture whereby the surface coating is continuously disrupted by the edges of the aperture during movement of the body to form a residue in the clearance space between the body and aperture to provide a liquidtight seal.

8. A method of growing single crystals of a fusible solid material from a melt which includes the steps of: preparing a bar of germanium material; applying to the bar a substanially continuous friable surface coating of silica; disposing said bar in a substantially vertical position; inserting the upper end of said bar into an open refractory vessel through an aperture in the bottom wall of said vessel; establishing a molten zone adjacent the upper end of said bar; and removing crystallized material from the molten zone while simultaneously progressively moving said bar relative to said vessel through said aperture and into the molten zone, said coating being continuously disrupted within said aperture during passage of said bar therethrough to form a residue between said bar and said aperture forming a liquid-tight seal.

9. A method of growing single crystals of la fusible solid material from a melt which includes the steps of: preparing a bar of germanium material; applying to the bar a substantially continuous friable surface coating of finely divided carbon material; disposing said bar in a su stantially vertical position; inserting the upper end of said bar into an open refractory vessel through an aperture in the bottom wall of said vessel; establishing a molten zone adjacent the upper end of said bar; and removing crystallized material from the molten zone while simultaneously progressively moving said bar relative to said vessel through said aperture and into the molten zone, said coating being continuously disrupted within said aperture during passage of said bar therethrough to form a residue between said bar and said aperture forming a liquid-tight seal.

10. A method of growing single crystals of a fusible solid material from a melt which includes the steps of: preparing a bar of silicon material; applying to the bar a substantially continuous friable surface coating of silica; disposing said bar in a substantially vertical position; inserting the upper end of said bar into an open refractory vessel through an aperture in the bottom wall of said vessel; establishing a molten zone adjacent the upper end of said bar; and removing crystallized material from the molten zone while simultaneously progressively moving said bar relative to said vessel through said aperture and into the molten zone, said coating being continuously disrupted within said aperture during passage of said bar therethrough to form a residue between said bar and said aperture forming a liquid-tight seal.

11. A method of growing single crystals of a fusible solid material from a melt which includes the steps of: preparing a bar of silicon material; applying to the bar a substantially continuous friable surface coating of finely divided carbon material; disposing said bar in a substantially vertical position; and inserting the upper end of said bar into an open refractory vessel through an aperture in the bottom Wall of said vessel; establishing a molten zone adjacent the upper end of said bar; and removing crystallized material from the molten zone while simultaneously progressively moving said bar relative to said vessel through said aperture and into the molten zone, said coating being continuously disrupted within said aperture during passage of said bar therethrough to form a residue between said bar and said aperture forming a liquid-tight seal.

References Cited by the Examiner UNITED STATES PATENTS 1,398,775 11/1921 Gerleman 117160 1,481,936 1/ 1924 Thomson 117-160 1,646,454 10/ 1927 Isaachsen 2330l 2,711,974 12/1951 Happe 117-127 2,871,143 1/1959 Getting 117-427 2,893,847 7/ 1959 Schweickert 23301 2,961,305 11/1960 Dash 23--301 2,985,519 5/1961 Kelemen 23301 3,036,188 3/1962 Wang 23-273 X OTHER REFERENCES Pfann: Zone Mel-ting, 1958, page 62.

NORMAN YUDKOFF, Primary Examiner.

GEORGE D. MITCHELL, ANTHONY SCIAMANNA,

MAURICE A. BRINDISI, Examiners.

G. P. HINES, A. J. ADAMCIK, A. KORON,

Assistant Examiners. 

1. IN COMBINATION WITH A METHOD OF PASSING A SOLID BODY INTO A LIQUID-FILLED VESSEL THROUGH AN APERTURE LOCATED BELOW THE LIQUID SURFACE, THE STEP OF APPLYING TO SAID BODY A FRIABLE SUBSTANTIALLY CONTINUOUS SURFACE COATING ADAPTED TO BE DISTRUPTED IN SAID APERTURE TO FORM A RESIDUE PARTIPATING IN THE FORMATION OF A SEAL BETWEEN SAID BODY AND VESSEL SAID COATING BEING SUBSTANTIALLY INSOLUBLE AND NON-REACTIVE IN THE LIQUID AND HAVING A MELTING TEMPERATURE EXCEEDING THE TEMPERATURE OF THE LIQUID. 